Update GenerateSchemaJsonHeader.ps1 to fix linker issues when including SchemaJson.h in multiple translation units

[chriche-ms]

The generated header SchemaJson.h has some issues:

1. Class SchemaJson should be a namespace
2. Schema strings (e.g. accessor_schema_json) don't follow the correct naming convention, and should either be defined in a separate .cpp or made constexpr const char[] to avoid linker issues.
3. Bracing style is not consistent with the rest of the codebase
4. GLTF_SCHEMA_MAP should be defined in a separate .cpp file.
5. Schema URIs shouldn't be hardcoded literals, instead move the relevant variables from Schema.h into the generated header.